An FRT flanked neo with a 5' loxP site was inserted upstream of exon 11. An artificial exon (11a) was created in intron 11 by inserting a cryptic splice donor site upstream of an existing cryptic acceptor site in intron 11. If spliced in, exon 11a creates a truncated CDS owing to an in-frame stop codon at the 5' end. A loxP site was inserted at the 3' end of exon 11a.